I need to clean the vacuum lines goin to and from the "ignition" switch
on my 1982 300D. What is the best way to get to that valve: from below,
removing the cover or from above, by removing the instrument cluster?

One good way to clean all of this system is to spray "brake clean " through the lines & the vacuum valve. W/key off disconnect brown & brown w/blue tracer vacuum lines(under hood) & spray brake clean through the lines. Should clean everything without disassembly.